Historically as fuel was being dispensed into a vehicle's fuel tank, typically from an underground storage tank (UST), vapor in the vehicle's fuel tank would escape into the atmosphere. In order to prevent this, Stage II vapor recovery systems were developed to collect this vapor and return it to the UST.
Stage II vapor recovery systems recover fuel vapor released from a vehicle's fuel tank as fuel is being dispensed into the vehicle's fuel tank. As is known, Stage II vapor recovery systems may be a balance type system or a vacuum-assist type system. Stage II vapor recovery systems typically are only installed in urban areas where the escaping fuel vapors can pose a greater threat to the environment.
In a further effort to prevent fuel vapors from escaping into the atmosphere in areas where Stage II vapor recovery systems are not prevalent, automobiles and subsequently light vehicle trucks, sold in the United States have been required to include an on-board refueling vapor recovery (ORVR) system, which is a vehicle emission control system that captures fuel vapors from the vehicle's gas tank during refueling. No fuel vapors escape from the fuel tanks of such ORVR equipped vehicles.
It is desirable to detect whether there is a partial or complete blockage in the vapor return path of a Stage II vapor recovery system. However it can be difficult to distinguish a blocked or otherwise restricted vapor return path from that of refueling an ORVR equipped vehicle.